The Toyota® Prius® is generally and presently regarded in the automotive world as a state-of-the-art high efficiency vehicle. In the Prius® drivetrain architecture, referred to hereinafter as the “Prior Art drivetrain” a naturally aspirated “Atkinson” Cycle gasoline internal combustion engine (ICE) is combined with two motor/generators and a planetary power split device (PSD). A single planetary gear set is used in an electrically-controlled Continuously Variable Transmission (eCVT). Control of the speed and torque of the two motor/generators has the effect of varying the gear ratio from the ICE to the output shaft through the PSD.
Although the Prior Art drivetrain offers a significant fuel economy improvement compared to current non-hybrid vehicles, especially on urban stop-and-go driving cycles, there are some notable drawbacks to its system.
First, the cost of the Prior Art drivetrain is significantly higher than that of a standard non-hybrid powertrain, because of the relatively high-power electric machines (also referred to herein as “motor/generators”), the associated power electronics, and the large high-power battery system.
Second, because of this same content plus an ICE having a relatively low power density, the Prior Art drivetrain weight is also much higher than that of a standard vehicle.
A trend in the automotive industry is the use of ICEs that have relatively low power (“downsized”) but are boosted by an exhaust-powered turbocharger using “free” energy. Such engines can reduce engine weight and improve fuel economy on typical light-duty driving cycles. Small-displacement direct-injection engines can have efficiency similar to that of the Prior Art drivetrain by using a turbocharger to boost torque and power.
Turbochargers are very efficient at constant high-speed high-load conditions where the volume and driving force of engine exhaust is also high. However, in lower speed stop-and-go driving, turbocharger efficiency is lower and there is no inherent ability in such engine configurations to use regenerative braking, as there is in a Prior Art drivetrain configuration. Engine torque tends to be low at and just above idle speed, resulting in low initial acceleration performance, non-linearity of torque response, and pollution challenges with transient emissions.
What is needed in the art is a lower-cost, lower-weight powertrain system for a vehicle that retains the attractive attributes of the Prior Art drivetrain, but which is compatible with the trend to downsized boosted engines, and which mitigates the poor low engine speed performance of prior art downsized turbocharged systems.
It is a principal object of the present invention to provide an improved powertrain system for a vehicle.